It is well known in the art to form metal powders and metal splats by pouring molten metal onto the top surface of a spinning disk which flings molten metal droplets outwardly into a quenching chamber and/or against a splat plate. The body of the atomizer disk is typically made from a high strength metal which can withstand the centrifugal loads at the high rotational speeds and temperatures to which it will be subjected. The bodies of atomizer disks are typically made from a high thermal conductivity metal, such as copper or a copper alloy which is water cooled to resist melting and/or erosion. Unfortunately, this results in an excessive amount of heat being removed from the metal being poured onto the disk, necessitating the use of large amounts of superheat (i.e., high molten metal pour temperatures) which can cause difficulties including possible melting at the center of the atomizer disk. It was also early on recognized that metals most suitable for forming the structural portion of the atomizer disk sometimes reacted with the molten metal being poured, thereby contaminating the metal powder being manufactured. The above problems intensify when atomizing metals which become highly reactive at high pour temperatures, or when the metal being atomized is an alloy having a large solidification range which requires even higher molten metal pour temperatures than would be required for atomizing the individual elements of the alloy.
One early solution to this problem involved lining the top surface of the metal atomizer disk with a refractory material, as shown in U.S. Pat. No. 2,439,772 to J. T. Gow. The refractory material, in addition to providing thermal protection for the underlying metal of the disk, was also felt to be inert or nonreactive to most molten metals. Even today the state-of-the-art of high speed rotary atomization for making powdered metal involves pouring the molten metal onto a ceramic layer which has been bonded to the surface of a metal atomizer disk, as is shown in U.S. Pat. Nos. 4,178,335 to R. A. Metcalfe and R. G. Bourdeau and 4,310,292 to R. L. Carlson and W. H. Schaefer, both owned by the assignee of the present application.
As discussed in hereinabove referred to U.S. Pat. No. 4,178,335, it is desirable, if not required, to form a solidified, stable "skull" on the ceramic surface of the atomizer disk of the metal being poured to get proper atomization. In the case of alloys having a large solidification zone, it is difficult and often not possible to obtain coupling between the ceramic disk surface and the molten alloy. In U.S. Pat. No. 2,699,576, to Colbry et al, magnesium is to be atomized on a steel disk (not ceramic coated). To achieve coupling Colbry et al adds zinc and zirconium to the magnesium.
Aluminum alloys and some other alloys having high concentrations of transition and other elements (i.e., Fe, Ni, Mo, Cr, Ti, Zr, and Hf) have very high melting temperatures and become very reactive toward many materials, including ceramics; and they also may possess a very large solidification range, in some cases over 500.degree. F., which prevents the formation of a skull or solidified layer on the surface of the atomizer. A number of other alloys, including off eutectic alloys of iron, copper, nickel and cobalt, belong to a class which also has a large solidification range and are therefore difficult to atomize properly. Other alloys, including the reactive metals chromium, titanium, zirconium, and magnesium, are a problem because of their high reactivity with materials, and especially if they are alloyed with elements which increase their melting points and increase their solidification range.
From the foregoing it becomes apparent that ceramic coated atomizer disks of the prior art have some shortcomings which have not been resolved.
The following additional U.S. patents are representative of the state-of-the-art in the field of rotary atomization: Nos. 4,069,045; 3,721,511; 4,140,462; 4,207,040; and Brit. Pat. No. 754,180.